1. Field of the Invention
This invention relates to a continuously variable transmission system for vehicles, which is constructed by combining a toroidal continuously variable transmission and a planetary gear mechanism.
2. Description of the Prior Art
Conventionally, a continuously variable transmission system for vehicles, of the above-mentioned kind, is disclosed e.g. in the publication of Japanese Patent No. 2778038. This continuously variable transmission system includes a toroidal continuously variable transmission, and first and second planetary gear mechanisms. The toroidal continuously variable transmission has an input shaft connected to an engine, and an output shaft connected to sun gears of the first and second planetary gear mechanisms. The first planetary gear mechanism has a planet carrier provided with a first clutch for connecting the planet carrier to a fixing member, and a ring gear connected to an output shaft of the continuously variable transmission system via a planet carrier of the second planetary gear mechanism. Further, the second planetary gear mechanism has a ring gear provided with a brake for reverse travel.
The toroidal continuously variable transmission has the input shaft thereof connected to an auxiliary drive shaft via two gears. The auxiliary drive shaft is connected to a first sleeve via two gears. The first sleeve is rotatably supported on the output shaft of the continuously variable transmission system. A second clutch is provided between the first sleeve and the ring gear of the second planetary gear mechanism. Further, the planet carrier of the first planetary gear mechanism is connected to a second sleeve via two gears. The second sleeve is rotatably supported on the auxiliary drive shaft. A third clutch is provided between the second sleeve and the auxiliary drive shaft.
In the continuously variable transmission system, when the vehicle is standing, the toroidal continuously variable transmission (hereinafter referred to as “the toroidal transmission” is controlled to a minimum speed transmission ratio (gear ratio), and the first to third clutches and the brake are disengaged or released to thereby hold the output shaft in a stationary state. From this state, when the first clutch is engaged to fix the planet carrier of the first planetary gear mechanism, the ring gear of the first planetary gear mechanism rotates in a direction opposite to the direction of rotation of the output shaft of the toroidal transmission, that is, in the same direction as the direction of rotation of the input shaft of the toroidal transmission, and the planet carrier of the second planetary gear mechanism and the output shaft of the continuously variable transmission system connected thereto also rotate in the same direction, whereby the continuously variable transmission system is placed in a first mode for forward travel. In this first mode, if the transmission ratio of the toroidal transmission is changed in a speed-increasing direction, the rotational speed of the sun gear of the first planetary gear mechanism is increased, and accordingly, the rotational speed of the ring gear of the same and hence the rotational speed of the output shaft of the continuously variable transmission system is increased, whereby the continuously variable transmission system delivers torque at an increased rotational speed.
Next, when the transmission ratio of the toroidal transmission reaches a maximum speed transmission ratio in the first mode, the first clutch is disengaged, and the second clutch is engaged, whereby the system is placed in a second mode for forward travel. In this second mode, part of torque of the input shaft is transmitted to the ring gear of the second planetary gear mechanism via the auxiliary drive shaft and the second clutch, whereby this ring gear is rotated in the same direction as the direction of rotation of the input shaft, and at the same time part of the torque of the input shaft is transmitted to the sun gear of the second planetary gear mechanism via the toroidal transmission, whereby this sun gear is rotated in a direction opposite to the direction of rotation of the input shaft. In this state, if the transmission ratio of the toroidal transmission is changed in a speed-decreasing direction, the rotational speed of the sun gear of the second planetary gear mechanism is decreased, and accordingly, the rotational speed of the planet carrier of the second planetary gear mechanism and hence the rotational speed of the output shaft of the continuously variable transmission system is increased, whereby the continuously variable transmission system delivers torque at a further increased rotational speed.
Then, when the transmission ratio of the toroidal transmission reaches the minimum speed transmission ratio in the second mode, the second clutch is disengaged and at the same time the third clutch is engaged, whereby the system is placed in a third mode for forward travel. In this third mode, part of the torque of the input shaft is transmitted to the planet carrier of the first planetary gear mechanism via the auxiliary drive shaft and the third clutch, whereby this planet carrier is rotated in the same direction as the direction of rotation of the input shaft, and at the same time part of the torque of the input shaft is transmitted to the sun gear of the first planetary gear mechanism via the toroidal transmission, whereby this sun gear is rotated in a direction opposite to the direction of rotation of the input shaft. In this state, if the transmission ratio of the toroidal transmission is changed in the speed-increasing direction, the rotational speed of the sun gear of the first planetary gear mechanism is increased, and accordingly the rotational speed of the ring gear of the same is increased, whereby the continuously variable transmission system delivers torque at a further increased rotational speed.
Further, if the brake is operated from the standing state of the vehicle, the ring gear of the second planetary gear mechanism is fixed and at the same time the sun gear of the same is driven for rotation by the output shaft of the toroidal transmission, so that the planet carrier of the second planetary gear mechanism and hence the output shaft of the continuously variable transmission system are rotated in the same direction as the direction of rotation of the output shaft of the toroidal transmission, that is, in a direction opposite to the direction of rotation of the input shaft of the toroidal transmission. This places the system in a reverse travel mode.
However, the conventional continuously variable transmission system necessitates two sets of planetary gear mechanisms, as described above, in order to realize the three (first to third) modes for forward travel. This increases the number of component parts of the system and manufacturing costs, and makes it impossible to design the system compact in size. Further, the system necessitates the reverse mode as a separate mode, in addition to the three modes. To implement the reverse mode, a brake for reverse travel is necessitated, which further increases the number of component parts and makes the system complicated in construction.
Another conventional continuously variable transmission system for vehicles, of the above-mentioned kind, has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 11-257449. This continuously variable transmission system is directed to prevention of excessive tilting of power rollers of a toroidal continuously variable transmission. This toroidal transmission includes an input disc rigidly fitted on an input shaft thereof, an output disc rotatably supported on the input shaft and arranged in a manner opposed to the input disc, and a pair of power rollers in abutment with mutually-opposed surfaces of the input and output discs. The pair of power rollers are supported on a pair of vertically extending trunnions, respectively, such that they are rotatable about a common roller axis orthogonal to the input shaft. Further, the pair of trunnions have upper ends and lower ends thereof supported by an upper link and a lower link, respectively, such that each trunnion is rotatable about a trunnion axis. Each trunnion is configured to be movable along the trunnion axis. By moving the pair of trunnions along the respective trunnion axes, the roller axis of the power rollers is displaced with respect to the rotational axis of the input and output discs, so that the pair of power rollers are rotated about the trunnions axes, respectively, by forces acting on the input and output discs and forces acting on the respective power rollers along the trunnions axes. As a result, the directions and angles of tilting of the pair of power rollers are controlled i.e. changed in a manner synchronous with each other, and the transmission ratio of the toroidal transmission is continuously changed according to the directions and angles of tilting of the pair of power rollers thus controlled.
Further, a pair of stoppers are arranged at respective predetermined locations close to each portion of the upper link for supporting an associated one of the trunnions so as to prevent the power rollers from being excessively tilted in a speed-increasing or speed-decreasing direction. Each trunnion is formed with a pair of receiving portions in a manner associated with the pair of stoppers. When the power rollers are tilted to a maximum speed position (OD end), one receiving portion of each trunnion is brought into abutment with the associated stopper of the upper link on the higher speed side, or alternatively when the power rollers are tilted to a minimum speed position (LOW end), the other receiving portion of the trunnion is brought into abutment with the associated stopper of the upper link on the lower speed side, whereby the angles of tilting of the power rollers are restricted to prevent the power rollers from being tilted beyond the maximum speed position and the minimum speed position to be detached from the input and output discs.
In the conventional continuously variable transmission system described above, however, the angle of tilting of the power rollers are restricted by causing the receiving portions of each trunnion to mechanically abut against the associated stoppers of the upper link, so that a large impact force acts on the upper link. Further, it is difficult to cause a plurality of trunnions to abut against stoppers simultaneously and uniformly due to variations in the machining accuracy, rigidity, assembling accuracy of component parts, which causes only one trunnion to be brought into abutment with the associated stopper. In such a case, an excessively larger force is concentratedly applied to the portion of the upper link supporting the trunnion. This makes it necessary to make robust the upper link and members associated therewith. Further, variations in torque transmitted by each power roller cause slippage of the power rollers, and the slippage causes abnormal generation of heat, early abrasion due to the heat, and the resulting degradation of durability. These inconveniences actually make it impossible to positively set the whole region of transmission ratios available from the continuously variable transmission to the range of transmission ratios for actual use, even if the stoppers are provided.